For a transmission screen to be used in microfilm reader, projection television, etc., attempts are made to increase light diffusion property of the screen. For this purpose, a lenticular lens, Fresnel lens, etc. are provided on light source side, and a light shielding layer is furnished on observer side to enhance screen contrast, and a light diffusion substance is added on the screen to increase light diffusion property of the screen.
Conventionally, to form such transmission screen, a synthetic resin sheet is produced by extrusion molding method, press molding method or casting method, and this is laminated with film of base material, and a lenticular lens or a Fresnel lens is formed directly on the film.
In the transmission screen, a Fresnel lens is arranged between various types of light sources such as cathode ray tube, liquid crystal filter type projection system on one side and a lenticular lens sheet on the other side, and light beam from the light source entering at high angle toward the surrounding region of the screen is converged slightly inwardly to prevent the darkening of the surrounding regions of the screen due to divergence of light from the light source.
When light beam enters Fresnel lens, refractive angle of the beam is determined according to the material of the Fresnel lens. The higher refractive index the material has, the less the light beam is lost even when the beam is refracted at high angle. Thus, if a material with higher refractive index is used for Fresnel lens, it is possible to refract light beam entering at high incident angle with less reflecting loss. As the result, it is possible to bring the position of the light source much closer to the screen and to shorten the distance between the light source of projection television and the screen. This contributes to designing of the projection television in smaller size.
Then, it is necessary to use a lens of short focal length as the Fresnel lens in order to shorten the distance between the light source of an image and the transmission screen. For this purpose, instead of methyl metacrylate resin with high transparency (refractive index n=1.49), a material of higher refractive index such as styrene is copolymerized with methyl metacrylate, and a sheet of the resultant material (e.g. having refractive index n=1.57) is used for the purpose by molding through thermal pressing.
However, the productivity of such Fresnel lens is not very high because it is produced by a series of processes including heating, pressurizing and cooling. In particular, to mold a Fresnel lens having fine lens shape over a large area by thermal pressing, it is necessary to heat under strict temperature condition and to perform pressurizing and cooling for long time, and this leads to very low production efficiency.
Under such circumstances, it has been proposed to utilize a UV-setting resin, which is cured by ionizing radiation such as ultraviolet ray within short time, instead of thermosetting resin as conventionally used in order to improve production efficiency and to form lenses with fine shape.
The UV-setting resin has a prepolymer of UV-setting resin as its material. The prepolymer of UV-setting resin is a macromolecular substance with low polymerization degree and high viscosity, and it is not possible to coat it on or to fill into a die. For this reason, it is necessary to use a diluent to dissolve the prepolymer. If an organic solvent is used as the diluent, the molded object is deformed due to evaporation of the organic solvent, and it is practiced to use a monomer as the diluent to polymerize with the prepolymer.
For a composition of UV-setting resin to be used in Fresnel lens or transmission screen using such lens must have, along with transparency of cured resin, high toughness not easily susceptible to rubbing as well as adequate surface hardness against deformation. As UV-setting resin usable for such purpose, there is urethane type acrylate resin. In order that the resin cured by ultraviolet ray maintains high refractive index, various types of acrylate resin effective to improve density of the resin are used in the monomer serving as diluent. However, the refractive index of a composition containing acrylate is not very satisfactory when compared with a material for thermal molding, in which a material with high refractive index such as styrene and methyl metacrylate are polymerized.
Also, in a two-sheet type transmission screen, it is practiced to combine a sheet where a Fresnel lens is formed with a sheet where a lenticular lens is formed, and further to transport the combined sheet and to incorporate it in the equipment for projection television. However, there is a problem that Fresnel lens surface is brought into contact with and damaged by surface of the other lens sheet due to vibration during transportation. In the past, such damage has been prevented by placing a buffer sheet between lens sheets or by adding a lubricant such as silicon oil. However, when the buffer sheet is placed, it is necessary to remove the buffer sheet by taking away the Fresnel lens sheet and the lenticular lens sheet. When lubricant such as silicone oil is used, the cost is increased and the resultant image becomes uneven due to uneven application of lubricant.
With the circumstances as such, there have been strong demands on a composition of UV-setting resin for transmission screen, which is made of a resin, having high refractive index, high transparency of cured resin, and high toughness against damage caused by rubbing.